A 58-year-old man with cirrhosis due to hepatitis B presents with a 2 cm nodule in the right lobe on ultrasound. MRI with hepatobiliary-specific contrast (gadoxetate disodium) is performed for HCC characterization. The structure marked **B** in the diagram is used as a key anatomical landmark to determine the extent of hepatic resection. Which of the following best describes the functional significance of the structure marked **B** in liver segmental anatomy?